1. Field of the Invention
The present invention relates to the touch screen technology, and more particularly, to the touch screen technology with Class A electromagnetic compatibility.
2. Description of the Prior Art
Electronic products emit electromagnetic radiation which usually causes one-way or mutual interference between each other. Designers and manufacturers of electronic products therefore must take into consideration the product's electromagnetic compatibility (EMC). EMC typically refers to the level of electromagnetic interference (EMI) resulting from electromagnetic radiation unintentionally generated, forwarded/relayed or received, to or from another electronic product.
Touch screen has become an important user-machine interface for the electronic products. Generally speaking, touch screens are usually equipped with a larger size of sensing electrodes. Proximity or touch events, also known as approximating or touching events, that take place on a touch screen are detected by measuring the weak currents flowing across the sensing electrodes. As a result, touch screen is vulnerable to electromagnetic interferences brought about through an external conductor, such as a contacting or approximating human body or stylus, or through the power or I/O lines.
The electromagnetic compatibility of an electronic product can be rated according to how the interference resulted from the noise is experienced by the user. If the user does not feel the presence of the occurring noise (thus the interference) at all, the compatibility is considered as Class A. If the noise can be detected but everything returns to normal again when it disappears, the compatibility is considered as Class B.
A typical capacitive touch screen includes an array of driving and sensing electrodes, where the row electrodes are orthogonal and exposed to the column electrodes. The multiple rows, for example, of driving electrodes are designed to emit impulses, the driving signals, which are sensed at a later time by the multiple columns of sensing electrodes. The driving signal is generated with respect to a particular working frequency that the timings of sensing and sampling by the sensing electrodes follow. External noises that come along with the human body, stylus, power lines or I/O lines are usually with respect to a certain frequency.
When the driving electrodes are not emitting the driving signals, the external interfering noise can be sensed by the sensing electrodes. If the noise frequency is closed to, identical or harmonic to the working frequency of the driving signal, the sensing electrodes' sampling and sensing of the driving signal will be severely interfered. In such case, the working frequency of the driving signal must be changed to avoid interference from the noise.
When the working frequency of the driving signal is changed, there are two possible consequences. Scanning of the touch screen at the new working frequency may generate sensing results different from those obtained prior to the frequency change, leading to displacements in the determined touch location. Or, the sensing electrode may simply stop reporting touch location information during the time period of changed working frequency. In either case, a broken touch trace could result when the user tries, for example, to draw a line using his finger. This causes the user's intended operation to fail. The user may succeed on a second attempt at a later time though. When this is the case, the electromagnetic compatibility of the electronic product would be determined as Class B. Solutions are desired to render Class A compatibility for the current capacitive touch screens, so a better user experience can be achieved.
From the above it is clear that prior art still has shortcomings. In order to solve these problems, efforts have long been made in vain, while ordinary products and methods offering no appropriate structures and methods. Thus, there is a need in the industry for a novel technique that solves these problems.